1. Field of the Invention
The present invention relates to a Lobro-type constant velocity universal joint that is suited for a propeller shaft used in four-wheel-drive cars and front-engine rear-wheel-drive cars.
2. Description of the Related Art
In four-wheel-drive cars and front-engine rear-wheel-drive cars, a so-called Lobro-type (or cross-groove type) constant velocity universal joint is used, to realize a propeller shaft construction that can cope with an angle change due to a change in the relative location between the transmission and the differential gear.
This Lobro-type constant velocity universal joint is shown in FIG. 4 as an example. This constant velocity universal joint has an inner ring 1, an outer ring 2, a ball 3, and a cage 4 as main components. The inner ring 1 has a stub shaft (not shown) of a propeller shaft intermeshed by serration onto its central hole 5, and has a plurality of track grooves 6 formed on its outer circumference surface, in the axial direction. The outer ring 2 is located on the outer circumference of the inner ring 1, and has the same number of track grooves 7 as the track grooves 6 of the inner ring 1, on its inner circumference surface, and in the axial direction. The track grooves 6 of the inner ring 1, and the track grooves 7 of the outer ring 2 are angled in the opposite direction with regard to the axis line, and the ball 3 is embedded in an intersecting portion between both of the track grooves 6 and 7. The cage 4 is arranged between the inner ring 1 and the outer ring 2, and the ball 3 is accommodated within a pocket 8 of this cage 4.
FIG. 5 is a cross-sectional view taken along the A—A line in FIG. 4, and the cage 4 and the outer ring 2 are not shown and only the inner ring 1 and the ball 3 are shown partially in an enlarged form. The cross-sectional shape of the track groove 6 is a Gothic arch form, formed by broaching processing with a radius of curvature R that is bigger than the radius “r” of the ball 3, as shown in the drawing. By making the cross section of the track groove a Gothic arch form, the contact between the track groove 6 and the ball 3 is made an angular contact, with a track contact angle α. It is the same with the track grooves 7 of the outer ring 2, which are not shown.
When an impact is generated in an automobile with this Lobro-type constant velocity universal joint, the components around the inner ring, such as the inner ring 1, the ball 3, and the cage 4, try to slide and move in the axial direction relative to the outer ring 2, through the intervention of the stub shaft of the propeller shaft that was subjected to the impact. A displacement between the transmission and the differential gear in the axial direction is absorbed by this slide movement, and the impact force inputted onto the car body through the intervention of the differential gear is reduced, substantially reducing the impact generated in the car body, and improving safety.
In this constant velocity universal joint, the cage 4 and the inner ring 1 slide in the axial direction, relative to the outer ring 2, when subjected to an impact in the axial direction caused by a car collision. Since the sliding resistance generating at this moment associates with vibration and noise problems in an actual car, or with durability problems caused by a rise in internal temperature in an actual car, the sliding resistance is strictly controlled as an important characteristic of the constant velocity universal joint. With the Lobro-type constant velocity universal joint used in a propeller shaft, the inner and outer rings 1 and 2, and the ball 3 are designed with the interference of the PCD clearance in mind, in order to eliminate backlashes inside the joint in the rotating direction, and the sliding resistance is determined by this interference of the PCD clearance.
This sliding resistance is strictly regulated for the whole area of the slidable area in the axial direction of the joint, or the whole area of the sliding area necessary in an actual car. The dispersion of the sliding resistance sometimes becomes high, because the amount of the aforementioned interference of the PCD clearance changes in accordance with factors such as heat treatment deformation of the track grooves 6 and 7 of the inner and outer rings 1 and 2, mutual pitch difference between the track grooves 6 and 7, and mutual difference between the intersecting angles. In this case, a matching operation will be needed in order to put the sliding resistance within a prescribed value.
The possibility of an occurrence of a deformation “a” or a burr “b”, caused by a dent, is high, particularly at the track groove 6 of the inner ring 1, as shown in FIGS. 6a and 6b. This is because, there is a possibility of the components colliding with each other during the transportation of the components, or during the input of components into processing machines, or during the ejection (falling) of the components, and also because the shoulder portion X connecting the aforementioned track groove 6 and the outer diameter portion of the inner ring 1, has an acute angle. Therefore, there is fear of a deformation “a” caused by dents generating at this shoulder portion X, and also fear of a burr “b” generating at the aforementioned shoulder portion X, when processing the track groove 6 by broaching. When there is a deformation “a” or a burr “b” caused by the dent, and when the ball 3 rotates and slides along the track groove 6, the ball 3 runs up onto this deformation or burr, and suddenly increasing the sliding resistance. This greatly affects the sliding resistance.